U.S. patent number 5,676,820 [Application Number 08/383,717] was granted by the patent office on 1997-10-14 for remote electrochemical sensor.
This patent grant is currently assigned to New Mexico State University Technology Transfer Corp.. Invention is credited to David Larson, Khris Olsen, Joseph Wang.
United States Patent |
5,676,820 |
Wang , et al. |
October 14, 1997 |
Remote electrochemical sensor
Abstract
An electrochemical sensor for remote detection, particularly
useful for metal contaminants and organic or other compounds. The
sensor circumvents technical difficulties that previously prevented
in-situ remote operations. The microelectrode, connected to a long
communications cable, allows convenient measurements of the element
or compound at timed and frequent intervals and instrument/sample
distances of ten feet to more than 100 feet. The sensor is useful
for both downhole groundwater monitoring and in-situ water (e.g.,
shipboard seawater) analysis.
Inventors: |
Wang; Joseph (Las Cruces,
NM), Olsen; Khris (Richland, WA), Larson; David (Las
Cruces, NM) |
Assignee: |
New Mexico State University
Technology Transfer Corp. (Las Cruces, NM)
|
Family
ID: |
23514401 |
Appl.
No.: |
08/383,717 |
Filed: |
February 3, 1995 |
Current U.S.
Class: |
205/777.5;
204/403.14; 204/403.15; 204/406; 204/412; 204/413; 204/434;
205/776.5; 205/787; 205/792; 205/793.5; 435/817 |
Current CPC
Class: |
G01N
27/42 (20130101); Y10S 435/817 (20130101) |
Current International
Class: |
G01N
27/49 (20060101); G01N 27/28 (20060101); G01N
27/42 (20060101); G01N 027/26 () |
Field of
Search: |
;204/403,406,412,413,434,153.1,153.12,404 ;435/817
;205/775,776.5,777.5,792,793.5,787 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 569 908 A2 |
|
Nov 1993 |
|
EP |
|
1 505 553 |
|
Mar 1978 |
|
GB |
|
WO 89/09388 |
|
Oct 1989 |
|
WO |
|
WO 91/08474 |
|
Jun 1991 |
|
WO |
|
WO 92/18857 |
|
Oct 1992 |
|
WO |
|
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Extraction and Processing of Mineral Resources, pp. 447-461 (1977)
Abstract no month available. .
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Fluorescence Spectrometer for Underwater Sediment Analysis,"
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available. .
Wring, S.A., et al., "Chemically Modified, Screen-Printed Carbon
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available. .
Yoneda, K.T., "Characteristics and Correlation of Various Radiation
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Niigata Prefectural Res. Lab for Health and Environ., pp. 156-162
(Oct. 1992) Abstract. .
Zauke, G.P., et al., "Biological Indicators of Environmental
Quality in the Elbe, Weser and Ems Estuary," Biologie
Umweltbundesamt, Berlin (Germany, F.R.), 156 p (Jul. 1986)
Abstract. .
Zirino, A., et al., "Measurement of Cu and Zn in San Diego Bay by
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12, No. 1, Abstract (Jan. 1978)..
|
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Peacock; Deborah A. Adams; Paul
Government Interests
GOVERNMENT RIGHTS
The Government may have rights to this invention pursuant to
Contract No. DE-FC04-90AL63805 awarded by the U.S. Department of
Energy Waste Management Education and Research Consortium (WERC).
Claims
What is claimed is:
1. A field environmental analysis apparatus for determining the
presence of pollutants in a natural environmental matrix
comprising:
a sensor assembly housed within an environmentally sealed connector
including multiple electrodes submerged in the natural
environmental matrix, one of said electrodes sensitive to the
presence of a pollutant;
a battery-powered analytical device;
a cable interconnecting said sensor assembly and said analytical
device; and
said analytical device including means for applying a periodic
voltage to the sensor assembly, means for receiving a response from
said sensor assembly, and means for converting, in real time, the
measured response to determine the presence of a pollutant.
2. The apparatus of claim 1 wherein said sensor assembly comprises
in addition to said pollutant sensitive electrode, a reference
electrode and a counter electrode.
3. The apparatus of claim 1 wherein said pollution sensitive
electrode is electrochemically reactive with the pollutant.
4. The apparatus of claim 3 wherein the pollution sensitive
electrode comprises a metal or carbon and the pollutant comprise a
metal.
5. The apparatus of claim 4 wherein the pollution sensitive
electrode comprises at least one member selected from the group
consisting of carbon and metal.
6. The apparatus of claim 3 wherein the pollution sensitive
electrode comprises an enzyme and the pollutant comprise an organic
compound.
7. The sensor of claim 6 wherein the electrode comprises
tyrosinase.
8. The apparatus of claim 6 wherein the pollutant is a phenolic
compound.
9. The apparatus of claim 1 wherein the pollution sensitive
electrode comprises at least one member selected from the group
consisting of fibers and arrays.
10. The apparatus of claim 1 wherein said cable comprises multiple
individually insulated metallic conductors, at least one conductor
for each electrode, a shield disposed around said conductors and an
outer insulation sheath.
11. The apparatus of claim 1 wherein said analytical device
includes means for cleaning the electrode, means for stripping the
electrode of a positive metal, and means for applying a negative
voltage to said pollution sensitive electrode to permit the
depositing of the metal pollutant thereon.
12. The apparatus of claim 11 wherein said means for cleaning the
electrode comprises means for applying a voltage of less than
approximately +1.0 V for a period of less than approximately five
minutes; said means for stripping metal comprises means for
applying a constant oxidation current in the range of approximately
0.1 to 5.0 .mu.A for a period of less than approximately five
minutes; and said means for collecting said metal pollutant samples
comprises means for applying a voltage between approximately -0.1
and -1.0 V for a period of less than approximately five
minutes.
13. The apparatus of claim 12 wherein the combined collecting,
stripping and cleaning functions are accomplished in less than
approximately 5 minutes.
14. The apparatus of claim 1 wherein the sensor performs
continuous, timed monitoring and analysis.
15. The apparatus of claim 14 wherein the sensor performs at least
approximately 15 runs/analyses per hour.
16. A method for on-site in situ environmental analysis utilizing
potentiometric stripping analysis to determine the presence of
pollutants comprising the steps of:
temporarily depositing a sensor assembly housed within an
environmentally sealed connector including multiple electrodes, one
of which is a pollution sensitive electrode, into a natural
environmental matrix at a depth below the surface of the
matrix;
locating a portable, battery-powered environmental analytical
device on the surface of the matrix;
connecting the electrodes of the sensor assembly through a multiple
conductor cable to the analytical device;
periodically applying a low-voltage signal from the analytical
device to the sensor assembly;
measuring the response to said signal from the electrode assembly;
and
continuously converting, in real time, the measured response to
determine the presence of a pollutant.
17. The method of claim 16 for determining the presence of metal
pollutants wherein the step of applying a voltage potential
includes the steps of:
cleaning the electrode for a period of less than approximately five
minutes at a voltage of less than approximately +1.0 V
depositing a metal constituent in the matrix by applying a voltage
of between approximately -0.1 and -1.0 V for a period of less than
approximately five minutes; and
stripping the deposited metal constituent by applying a constant
oxidation current, in the range of approximately 0.1 to 5.0 .mu.A,
for a period of less than approximately five minutes.
18. The method of claim 17 wherein the cleaning, collecting, and
stripping steps are repeated at least approximately fifteen times
per hour.
19. The method of claim 16 additionally comprising the steps of
connecting at least one of the multiple conductors to each sensor
and insulating the signals on the conductors from ambient
electrical noise.
20. The method of claim 16 for determining the presence of organic
compound pollutants and comprising the additional steps of:
applying an enzyme contained in a paste to the pollutant sensitive
electrode;
periodically applying a voltage potential; and
measuring the resultant current transients to thereby measure
quinone products of environmentally relevant phenolic
substrates.
21. The method of claim 16 including insulation of the current
transient response of the sensor from environmental electrical
noise.
22. The method of claim 16 wherein the applied voltage is changed
in steps from approximately +0.2 V to -0.1 V.
23. The method of claim 22 wherein the step of applying the voltage
is performed at intervals of approximately 15 minutes or less.
24. The method of claim 16 wherein the steps of periodically
applying a low voltage signal and measuring the response is
continuous.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention--Metals
The present invention relates to remote, on-site, continuous
monitoring of metals and organic compounds, particularly phenolic
compounds.
2. Background of Invention--General
Detection and monitoring of metals and organic compounds is
normally done by having an operator collect on-site field samples
and then taking the samples back to a laboratory.
Attempts have been made to provide both remote field sampling and
analysis, on-site. See U.S. Pat. No. 5,120,421, entitled
Electrochemical Sensor/Detector System and Method to Glass et al.,
and U.S. Pat. No. 5,296,125, entitled Electrochemical Sensor
Detector System and Method to Glass et al. However, an operator is
still required to be present on-site to collect the sample, and the
device does not communicate directly with the laboratory.
3. Background of Invention--Metals
Contamination of hazardous waste sites and groundwater with toxic
heavy metals (e.g. Hg, Pb, U, As, Cr, Al) represents a major
national problem. Site monitoring and surveillance programs are
required for a closer control of metal pollutants. The traditional
use of atomic-spectroscopy central-laboratory measurements of heavy
metals is too expensive and time consuming. Also, samples often
change composition during their collection, transport and delay
before analysis, ultimately producing unreliable results.
Innovative field deployable methods are highly desired for the task
of site characterization and remediation, as they minimize the huge
labor analytical costs, and provide timely data for real-time
emergencies and decision making. Chemical sensors are particularly
attractive for providing real-time, remote monitoring of priority
pollutants. While fiber-optic probes have been suggested for
monitoring organic contaminants, no chemical sensor technology has
demonstrated capability for remote monitoring of trace metals. (See
W. Chudyk, et al., J. Anal. Chem. 1985, 57, 1237.) Clearly, a cost
effective metal-sensor technology, capable of monitoring the metal
both in time and location, is needed to support the
characterization and remediation of hazardous waste sites. (See G.
Batiaans, et al., (Eds.), "Chemical Sensors: Technology Development
Planning," U.S. Department of Commerce, Springfield, 1993.)
In the present invention, there is provided a remote sensor for
in-situ monitoring of trace metals. The compact instrumentation and
low power needs of electrochemical techniques satisfy many of the
requirements for on-site metal analysis. Particularly attractive
for in-situ monitoring of metal contaminants is the remarkably
sensitive technique of stripping analysis. (See J. Wang, "Stripping
Analysis: Principles, Instrumentation, and Applications," VCH
Publishers; Deerfield Beach, Fla., 1985.) The extremely low
(subnanomolar) detection limits of stripping analysis are
attributed to its "built-in" preconcentration step, during which
the target metals are deposited onto the working electrode.
The feasibility of using stripping analysis for field-based
operations was demonstrated first by the U.S. Navy, who developed
an automated flow system, based on a mercury-coated open tubular
electrode, for continuous shipboard monitoring of trace metals in
oceans. (See A. Zirino, et al.; C. Environ. Sci. Technol., 1978,
12, 73.) Another useful flow system (based on a hanging mercury
electrode) was deployed by Buffle's group for in-situ metal
monitoring in lakes and oceans. (See M. Terrcier, et al., J.
Electroanalysis, 1993, 5, 187.) Yet, submersible (downhole) sensors
based on stripping analysis have not heretofore been developed due
to several technical difficulties (e.g., use of mercury surfaces,
needs for oxygen removal, and solution stirring or supporting
electrolyte) which prevent continuous remote operations.
The field probe of the present invention addresses these challenges
by combining several stripping methodologies. In the preferred
embodiment, these include the replacement of the traditional
mercury electrodes with gold surfaces (see J. Wang, et al.,
Electroanalysis, 1993, 5, 809; J. Wang, Anal. Chim. Acta, 1994,
286, 198; E. P. Gil, et al., Anal. Chim. Acta, 1994, 293, 55), the
use of ultramicroelectrodes which offers efficient mass transport
(independent of natural convection) as well as work in low
ionic-strength natural waters (see J. Wang, et al., J. Electroanal.
Chem., 1989, 246, 297; J. Huiliang, et al., Anal. Chem. Acta. 1987,
193, 61) and the incorporation of potentiometric stripping analysis
(PSA) which eliminates the need for oxygen removal and minimizes
the surfactant effects (see D. Jagner, Trends Anal. Chim. Acta,
1993, 273, 35; P. Ostapczuk, Anal. Chem. Acta, 1993, 273, 35). Such
combination of gold ultramicroelectrodes with PSA thus facilitates
remote operations by making solution stirring or deoxygenation,
electrolyte addition or mercury electrodes unnecessary. These
electrochemical considerations are coupled with the need for a
compact and rugged probe, hence ensuring that the in-situ sensor
performs comparable to established laboratory-based stripping
instruments.
4. Background of Invention--Phenolic Compounds
Because of the inherent toxicity of phenolic and other organic
compounds, there is a considerable interest in their determination
in environmental matrices. Such assays in environmental samples are
usually carried out in central laboratories using separation
techniques such as liquid chromatography. However, in view of the
huge labor and analytical costs or time delays associated with
centralized laboratory analyses, there are immediate needs for
developing field sensors for organic compounds. A real-time
continuous monitoring capability, in particular, should give rapid
warning in case of sudden contamination, provide rapid feedback
during site remediation activities, and be valuable for studying
processes in an aquifer. Field sensors for organic compounds thus
greatly improves the quality and efficiency of site monitoring or
remediation activities.
Tyrosinase-based enzyme electrodes have been shown useful for the
selective determination of phenols in environmental matrices (see
M. Bonakdar, J. Electroanal. Chem. 1989, 266, 47; J. Wang, et al.,
Analyst, 1994, 119, 455; L. Campanella, et al., Analyst, 1993, 118,
979; F. Ortega, et al., J. Pharm. Biomed. Anal., 1992, 10, 789).
Such devices commonly rely on the reductive amperometric detection
of the liberated quinone species. For this purpose, tyrosinase is
immobilized onto the transducer surface or incorporated (mixed)
within a carbon paste matrix. However, enzyme-based remote
electrodes, capable of making continuous real-time measurements at
large sample/instrument distances, have not been reported.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
The present invention relates to electrochemical sensing. The
preferred sensor and method of the invention comprises an electrode
assembly for contacting at least one constituent in a remote
location, a long (10 or fewer to more than 100 feet) communications
cable for connecting the electrode assembly from the remote
location to an analysis location, and an analysis device for
analyzing constituent information obtained from the electrode and
communicated to the analysis device via the long communications
cable.
The preferred electrode assembly comprises a three-electrode
assembly; a working electrode, a reference electrode and a counter
electrode. The electrode is electrochemically reactive with the
constituent. The electrode preferably comprises a metal (e.g. gold
or silver) or carbon when the constituent comprises a metal. The
electrode preferably comprises an enzyme (e.g. tyrosinase) when the
constituent comprises an organic compound (e.g. a phenolic
compound). The electrode may comprise a fiber or arrays or other
structures.
The sensor preferably comprising preconcentration and stripping
functions and additionally a cleaning function. All of these
functions are preferably accomplished in less than 5 minutes. The
sensor performs continuous, timed monitoring and analysis (e.g. 15
runs/analyses per minute).
A primary object of the present invention is to provide remote and
continuous monitoring and sensing of metals and organic
compounds.
A primary advantage of the present invention is that remote on-site
accurate monitoring can be made without requiring continuous
on-site field personnel.
Other objects, advantages and novel features, and further scope of
applicability of the present invention will be set forth in part in
the detailed description to follow, taken in conjunction with the
accompanying drawings, and in part will become apparent to those
skilled in the art upon examination of the following, or may be
learned by practice of the invention. The objects and advantages of
the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a
part of the specification, illustrate several embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. The drawings are only for
the purpose of illustrating a preferred embodiment of the invention
and are not to be construed as limiting the invention. In the
drawings:
FIG. 1 is a front and schematic view of the preferred metal sensor
of the present invention;
FIG. 2 shows graphs of the stripping potentiometric response of the
sensor of FIG. 1 to untreated seawater (A) and groundwater (B)
containing metal contaminants;
FIG. 3 shows graphs of the stripping potentiometric response of the
sensor of FIG. 1 to groundwater solutions containing increasing
levels of (A) mercury and (B) selenium;
FIG. 4 shows graphs of the stability of the stripping
potentiometric response of the sensor of FIG. 1 to groundwater
solutions during a long run of 20 successive measurements of copper
(A) and lead (B);
FIG. 5 shows graphs of the ability of the sensor of FIG. 1 to
follow sudden changes in concentrations of copper (A) and lead
(B);
FIG. 6 shows graphs of the response of the sensor of FIG. 1 in
groundwater (A) and seawater (B);
FIG. 7 is a front and schematic view of the preferred biosensor of
the present invention;
FIG. 8 shows graphs of the chromoamperometric response of the
sensor of FIG. 7 to cresol (A) and phenol (B);
FIG. 9A and 9B show graphs of the influence of pH on the sensor of
FIG. 7;
FIG. 10 shows a graph of the dependence of the operating potential
upon the FIG. 7 sensor response to phenol;
FIG. 11 shows graphs of the FIG. 7 sensor response to untreated
river water (A) and groundwater (B); and
FIG. 12 shows graphs of the response of the FIG. 7 sensor over
prolonged period to a constant concentration of phenol (a) and to
dynamic changes in phenol concentration (b).
DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING
OUT THE INVENTION)
The present invention provides for remote sensing. The remote
sensors can detect metals or organic compounds in-situ in a wide
range of natural waters and aqueous industrial process streams.
The present invention further provides for a portable, battery or
electrically operated, fully automated microprocessor-controlled
instrument, capable of unattended operation (in the stripping or
amperometric modes), "smart" data processing and signal
transmission to a laboratory or analysis center.
METAL SENSOR
The probe design of the invention for remote sensing of metals aims
at addressing both electrochemical and environmental requirements,
namely the achievement of an optimized stripping performance, as
well as compatibility with remote field work (e.g., downhole or
shipboard or other water monitoring). The electrochemical
considerations are thus coupled with the need for a rugged device
and rapid maintenance (e.g., fast replacement of electrodes).
The preferred sensor, shown in FIG. 1, comprises an electrode
assembly 10 connected (through environmentally-sealed connectors
12) to a long, shielded communications cable 14 (e.g. 10 feet to
more than 100 feet). The shielded cable 14 assures a negligible
electrical noise even for large instrument/sample distances (e.g.
hundreds of feet). The microelectrode 16, where the
deposition/stripping reactions of interest take place, represents
the heart of the probe. This microelectrode 16 is preferably made
of gold fiber, but other metals, carbon and
structures/configurations (e.g., screen printed or
lithographically-made strip, wire, band, arrays, deposit or film on
a substrate, etc.) can be utilized in the invention. The transducer
surface of the microelectrode 16 is chosen based on the metal to be
detected. Gold surfaces offer convenient quantitation of
environmentally-relevant trace metals (such as copper, lead,
selenium, arsenic, or mercury). Although one advantage of the
present invention is to replace mercury-based electrodes, mercury
systems can be utilized in accordance with the invention, depending
on the metal being detected. Measurements of lead and copper with a
gold microelectrode compare favorably with those observed at
mercury-based sensors. The microelectrode configuration obviates
the need for solution stirring during the deposition step, and
allows measurements with little (or no) supporting electrolyte. Its
coupling to a PSA operation also eliminates the need for a
time-consuming deoxygenation step. Such simplified operation thus
meets many of the requirements for remote chemical sensing.
The electrode assembly sensor tip comprises three electrodes, the
working microelectrode 16 (again, preferably gold fiber), a
reference electrode 18 and a counter electrode 20. These three
electrodes 16, 18, 20, are connected to the long shielded cable 14
via an environmentally-sealed connector 22 (e.g. silicone or
rubber). Electrode sensing information is then communicated via the
long shielded cable 14 to an analysis device 34 (e.g. a
voltammograph).
In the preferred embodiment, the working electrode 16 and reference
electrode 18 are sealed into fittings 24, 24', disposed in coupling
connectors 26, 26' which are further disposed in housings 28, 30.
This arrangement provides for easy removal of these electrodes 16,
18. An additional housing 32, provides further stability.
Real time, continuous monitoring is possible using the probe of the
present invention. The analytical utility of the remote probe is
based on the linear correlation between PSA response and the target
metal or contaminant concentration. Only several minutes (e.g. 3-5)
are required for the entire deposition/stripping/cleaning cycle.
Numerous tests can therefore be made (e.g. 15-20 runs per
hour).
The invention is further illustrated by the following non-limiting
examples.
EXAMPLE I
Experimental Results--Metals
A three-electrode assembly (100-.mu.m diameter, 5-mm long gold
microcylinder working electrode, silver-silver chloride reference
electrode (BAS Model RE-4) and a platinum wire counter electrode
contained in a PVC housing, was connected to a long shielded cable,
via a 3-pin environmentally-sealed rubber connector. The gold
microelectrode was fabricated as described in e.g. J. Wang, et al.,
Anal. Chim. Acta, 1994, 286, 189. Both the working and reference
electrodes were sealed into Teflon.RTM. fittings, screwed into
female coupling connectors, which were fixed with epoxy in the PVC
housing. The counter electrode was fixed permanently into the
housing with epoxy. Electrical contact to the working and reference
electrodes was accomplished with the aid of brass screws and spring
assemblies, contained inside 7-mm o.d. copper tubes. The latter
were placed within the female connectors, and were soldered to the
copper wire contact. The other end of the copper wires was
connected to the male environmentally-sealed connector. Another
copper wire was used to connect the platinum electrode to the
environmentally-sealed connector. The entire assembly was sealed
into the PVC housing. The 3-pin male connector (Newark Electronics)
was connected to the receptable, which was attached to the shielded
cable. Such connection permits quick disconnection of the electrode
housing from the cable. The female connector was sealed in a PVC
tube that provides additional stability. A 3-cm diameter PVC tube
was threaded onto the housing unit to protect the electrode
assembly during field testings. Cables of different lengths,
ranging from 25 to 100 ft., were employed in accordance to the
specific field application.
Apparatus
Potentiometric stripping experiments were performed using a
TraceLab potentiometric stripping unit (PSU 20, Radiometer,
Copenhagen), in connection with an IBM PS/55SX or Thinkpad Laptop
computers. The manual instrument used in the examples was compact,
10.times.6.times.3 inches, and light weight (8 lb). Some
experiments were carried out with a smaller
microprocessor-controlled "homemade" PSA analyzer.
Reagents
All solutions were prepared with double-distilled water. Stock
metal solutions were prepared daily from the corresponding 1000 ppm
atomic absorption standards (Aldrich). Experiments were carried out
in unpreserved groundwater (from Hanford Site, Richland, Wash.) or
seawater (San Diego Bay, Calif.) solutions without additional
supporting electrolyte.
Procedure
Laboratory experiments and field pre- or post-calibrations were
performed in a 400 mL Erlenmeyer flask into which the entire probe
was immersed. A silicone-grease was applied to all connectors (to
ensure sealing needed for preventing entry of solution). Prior to
the deposition step, the gold electrode underwent a "cleaning"
procedure for 1-2 min at +0.5 to +1.0V (depending on the target
metals). Deposition proceeded in the quiescent solution for 1-2 min
at potentials ranging from -0.3 to -0.7V (depending upon the
sought-for metal(s)). After the deposition period, the potentiogram
was recorded by applying a constant oxidation current (in the
0.3-2.0 .mu.A range) and the BASE 3 or 4 commands of the Trace Lab
software for baseline treatment. A similar PSA protocol was
employed when measurements were made in the field (for in-situ
monitoring of seawater or groundwater).
FIG. 2 displays the stripping potentiometric response of the sensor
to unpreserved seawater (A) and groundwater (B) samples, spiked
with 5-10 .mu.g/L (ppb) levels of lead, copper and mercury.
Well-defined and sharp peaks are observed, despite the use of
nondeaerated samples, unstirred solutions or short (1-2 min)
deposition periods. The well-resolved peaks (.DELTA.Ep>0.2V) and
the flat baseline allow convenient multielement determinations of
low .mu.g/L concentrations in these unpreserved samples. The exact
peak potentials (around -0.2V (Pb), +0.2V (Cu) and +0.45V (Hg))
depend on the extent of binding to complexing agents present in
these samples. The copper peak (in the seawater medium) increased
linearly with the preconcentration time (up to 10 min), while the
lead peak increased sigmoidally upon changing the deposition
potential between -0.3V and -1.0V (with a plateau reaching above
-0.5V (not shown)). Overall, a deposition potential of -0.5V
allowed simultaneous measurements of all three metals, while a
lower one (-0.3V) was preferred for the copper-mercury pair. (More
positive deposition potentials extend the anodic potential window,
as desired, for measurements of metals with relatively positive
stripping potentials.) Analogous stripping voltammetric
measurements (using the rapid square-wave mode) yielded an inferior
performance (with convenient quantitation only above the 10 .mu.g/L
level) due to a higher baseline response (not shown). The
computerized PSA instrument is superior in addressing background
contributions associated with the gold surface. Yet stripping
voltammetry may also be feasible for operation at the .mu.g/L
concentration range.
FIG. 3 displays stripping potentiograms, following one-min
deposition, for unstirred groundwater solutions containing
increasing levels of (A) mercury and (B) selenium (in 20 .mu.g/L
steps). These peaks are part of a series of ten concentration
increments over the 10-100 .mu.g/L range. The well-defined peaks
increase linearly with the metal concentration over the entire
range. Linearity up to 100 .mu.g/L was observed also in analogous
calibration experiments for lead and copper (not shown). The
dynamic range of the probe can be changed by adjusting the
deposition time. Detection limits of 0.7 .mu.g/L copper and 1.1
.mu.g/L lead were estimated from the response, following one-min
deposition, for a groundwater sample containing 5 and 10 .mu.g/L of
these metals (e.g., see FIG. 2B). Even lower detection limits can
be achieved by further extending the deposition time. Overall, ca.
3-4 min are required for the entire deposition/stripping/cleaning
cycle (for quantitation at the 1-20 .mu.g/L range), thus leading to
near real-time monitoring at a rate of 15-20 runs/hour.
High measurement stability is an important requirement for the
remote in-situ probe. The short "cleaning" step insures complete
removal of the deposited metal at the end of each run, and hence a
"fresh" (analyte-free) gold surface prior to the next measurement.
Hence, the deposition/stripping cycle leads to a reversible sensor
behavior. FIG. 4 examines the stability of the response during a
long run of 20 successive measurements of copper (A) and lead (B)
in groundwater. A highly stable response is observed over these
prolonged (50-70 min) operations. The relative standard deviations
for these complete series were 2.1% (A) and 3.2% (B). High
stability was also observed in longer (3-5 hours) field testings,
described below. The stable response in unpreserved natural water
samples is also attributed to a minimal surfactant effect
associated with the use of PSA and gold surfaces. Whenever needed,
the probe construction permits a fast and easy replacement of
electrodes. A proper sealing of all connectors (e.g. by coverage
with silicone grease) is essential to achieve such stable response,
through prevention of liquid entry into electrical contacts.
The ability of the remote probe to follow sudden changes in the
concentration of copper and lead is illustrated in FIG. 5. The
temporal profile, shown in (A), resembles a sharp contaminant
release, with 10 runs in a 10 .mu.g/L copper solutions, followed by
three replicates in a 40 .mu.g/L solution, and 10 measurements at
the original 10 .mu.g/L level. The response rises rapidly from the
10 .mu.g/L baseline upon immersing in the 40 /.mu.g/L solution, and
decays sharply upon returning to the original low-concentration
medium. Similarly, minimum carry-over effects are indicated from
(B), involving three replicates at the 20 .mu.g/L lead level, 10
runs in a 10 .mu.g/L solution, and return to the original 20
.mu.g/L one. These data show how precalibration and/or
postcalibration techniques can be utilized with the same protocol
employed during the in-situ runs.
Several on-site demonstrations were successfully performed,
including a downhole groundwater monitoring and an in-situ
shipboard seawater analysis. FIG. 6 shows a stripping potentiogram
for groundwater (A) and seawater (B).
The groundwater well (A) was at the Hanford Site (Richland, Wash.).
Such in-situ PSA response was obtained at a depth of 40 ft., and is
a part of a continuous 3h downhole operation at different depth
intervals (35-55 ft). A small copper peak (at ca. +0.25V) was
observed in these repetitive runs. Such response corresponds to 3
.mu.g/L, as was confirmed in subsequent ICP/MS measurements. No
apparent electrical noise was observed despite the significant
cable length.
FIG. 6(B) illustrates the response of the in-situ probe for
seawater in San Diego Bay (California). Such measurement was made
from a small boat, with the probe dangling from the side of the
vessel just below the water surface. It was part of a 5-hour
in-situ study at different locations in San Diego Bay. On-boat
precalibration experiments, employing a spiked seawater sample,
were used for computing the concentration. Large copper and lead
peaks, corresponding to 11 and 2 .mu.g/L, respectively, were
observed. Such relatively high levels were anticipated for this
specific location (the south embayment, "pocket," of Shelter
Island), where discharge from pleasure boats and restricted
circulation exist. Substantially lower copper and lead levels (of
0.9 and 0.7 .mu.g/L, respectively) were detected at the mouth of
the bay (around Point Loma), as expected for the metal distribution
in San Diego Bay (see A. Zirino, et al.; C. Environ. Sci. Technol.,
1978, 12, 73). Such values indicate that contamination from the
boat did not contribute to the sensor response.
The above examples illustrate the utility of stripping-based
electrochemical sensors for remote monitoring of trace metals. The
remote probe offers significant advantages for in-situ measurements
of priority metal contaminants, including remarkable sensitivity,
multielement and speciation capabilities, high selectivity, small
size and low cost. Such coupling of metal ultramicroelectrodes with
PSA overcomes difficulties associated with the adaptation of
traditional stripping procedures to long-term remote operations.
Additional stripping procedures (e.g. absorptive stripping) or
ligand-modified electrodes can be incorporated for monitoring
additional analytes. The probe can be further miniaturized to
permit adaptation to cone penetrometer technology.
ORGANIC COMPOUND SENSOR
In another embodiment of the invention, for detecting organic
compounds, the preferred sensor addresses key environmental and
electrochemical requirements, namely the achievements of an
optimized biosensing performance and compatibility with remote
field work. The preferred remote enzyme sensor 50 is shown in FIG.
7. The sensor 50 includes a specially designed bioelectrode
assembly 52 (comprising an enzyme electrode 58 a reference
electrode 60 and a counter electrode 62), connected (via
environmentally sealed connectors 54 (similar as shown in FIG. 1))
to a long (e.g., 10 feet to more than 100 feet) shielded
communication cable 54 and then to an analysis device 56 (e.g. a
voltammograph).
The preferred probe 50 comprises a specially designed (preferably
tyrosinase) enzyme electrode (e.g. tyrosinase for phenol
measurements) coupled to the shielded cable 54. The combination of
biocatalytic recognition and amperometric transduction offers
highly selective measurements of micromolar concentrations of
organic compounds in unpreserved river and groundwater samples.
Remote monitoring of the compounds is provided via a judicious
selection of the biorecognition element. Such elements include an
enzyme, antibody, whole cell or receptor.
The biosensing considerations are thus coupled with the need for
rugged and compact devices, large instrument-sample distances and
rapid maintenance. In addition, operational conditions are
optimized to meet the specific requirements of remote
operation.
While the examples illustrate a remote enzyme electrode presented
within the framework of phenol sensing, it can be utilized to
detect other relevant pollutants through a proper choice of the
biocatalyst. Similar enzyme-based probes can be utilized for other
pollutants (e.g., sulfite, peroxide, formaldehyde) via a choice of
the biocomponent. Different enzyme tips may be mounted on the same
probe to allow simultaneous detection of several contaminants.
There are several technical challenges associated with the
adaptation of tyrosinase electrodes, and electrochemical biosensors
in general, to remote environmental sensing. Unlike
laboratory-based biosensing applications, where the solution
conditions can be adjusted for optimal performance, remote
operations rely on the use of the natural conditions (e.g., pH,
ionic strength, convection). In addition, the in-situ continuous
probe of the present invention offers a stable and fast response,
with no apparent carry over.
EXAMPLE II
Experimental Results--Phenolic Compounds
The probe consisted of a three-electrode assembly (in a 25-mm i.d.
PVC housing tube), connected to a 50-ft long shielded cable, via a
3-pin environmentally sealed rubber connector (Newark Electronics).
Two female coupling connectors, fixed with epoxy in the PVC tube,
served for mounting the enzyme electrode and silver-silver chloride
reference electrode (Model RE-4, BAS). These electrodes were sealed
into Teflon male fittings, hence allowing their easy and fast
replacement. A platinum wire counter electrode was fixed
permanently into the housing. Contact to the enzyme and reference
electrodes was made via brass screws and spring assemblies,
contained inside 7-mm o.d. copper tubes. The latter were placed
within the female connectors, and were soldered to copper wire
contacts. The other end of these wires was connected to the male
environmentally sealed connector. A similar contact was made to the
platinum counter electrode. The male connector was coupled to the
receptacle, which was attached to the shielded cable. Such
arrangement allowed for rapid disconnection of the electrode
housing from the cable. The female connector was also sealed in a
PVC tube that provided additional rigidity.
Apparatus and Procedure
Experiments were performed with a Bioanalytical systems (BAS) Model
CV-27 voltammetric analyzer, in connection with a BAS X-Y-t
recorder. The manual instrument used in the examples was compact,
10.times.6.times.3 inches, and light weight (8 lb). The electrode
housing was immersed in the sample solution, contained in a 300 ml
beaker, and located 50 feet from the voltammetric analyzer.
Chronoamperometric experiments were conducted by applying a
potential step (to O.OV) and recording the resulting current
transient. Quantitative information was obtained by sampling the
current after 60 second. All experiments were carried out at room
temperature.
Carbon Paste Preparation
The enzyme-containing paste was prepared by hand mixing first 10 mg
(42,000 units) of tyrosinase with 4 mg of a polyethylenimine (PEI)
stabilizer. This mixture was added to 186 mg of unmodified carbon
paste (65% w/w Acheson 38 graphite powder/35% w/w Aldrich mineral
oil). Mixing proceeded for 30 minutes. A portion of the resulting
paste was then packed firmly into the cavity of the working
electrode body (BAS, Model MF-2015). The paste surface was smoothed
on a weighing paper.
Reagents
All solutions were prepared with deionized water. Tyrosinase (from
mushroom, EC 1.14.18.1, 4,200 units/mg) was utilized from Sigma
(catalog number 5-7755). Phenol was received from J. T. Baker,
while p-cresol was purchased from Aldrich. Tests were carried out
using a phosphate buffer solution (pH 5.5, 5 mM) and unpreserved
Hanford-Site groundwater and Rio Grande river water (collected at
Richland, Washington and Las Cruces, N. Mex., respectively).
Procedure
The tyrosinase carbon paste electrode is where the biocatalytic
recognition took place. The immobilized tyrosinase catalyzed the
o-hydroxylation of phenolic compounds to catechols, with subsequent
dehydrogenation to o-quinones. Low potential detection of the
quinone product thus allowed convenient measurement of
environmentally-relevant phenolic substrates down to the
submicromolar level.
FIG. 8 displays the chromoamperometric response of the remote
sensor to cresol (A) and phenol (B) solutions of increasing
concentration (in 1.times.10.sup.-6 steps, b-k). The probe
responded favorably to these micromolar concentration changes.
Detection limits around 2.times.10.sup.-7 M cresol and
3.times.10.sup.-7 M phenol were estimated based on the favorable
signal-to-noise characteristics and the very low background
response. Such low background current was attributed to the low
(O.OV) operating potential. In addition, no apparent noise
contribution from the connecting cable was observed (despite the 50
foot long solution/instrument distance). Detection limits were thus
similar to those of conventional (non-remote) tyrosinase
electrodes. Further lowering of the detection limits (down to the
low nanomolar level) can be achieved in connection with
bioamplification schemes.
The sample/instrument distance (i.e., cable length) had no effect
upon the response to phenol or the corresponding noise level.
Similar response characteristics were observed using distances of
25, 50 and 100 feet. Indeed, the performance with such cables was
similar to that observed (with the same electrode housing) without
the cable.
The influence of the pH upon the response of the remote electrode
is displayed in FIG. 9(A). High sensitivity was observed over a
broad pH range (4.5-6.5), relevant to most natural water samples,
with decreased response at higher pH values. Such pH profile
reflects the broad pH activity of tyrosinase (see G. Rivas, et al.,
Bioelectrochem. Bioenerg., 1992, 29, 19). FIG. 9(B) shows the
effect of the buffer concentration upon the sensor response. A
similar sensitivity was observed between 5 and 50 mM (and at higher
levels (not shown)). About 40% and 70% signal losses are observed
at 1- and 0 mM buffer, respectively.
Minimal dependence on mass transport is another useful property of
the tyrosinase-based sensor. For example, only a 15% increase in
the 5.times.10.sup.-6 M phenol response was observed upon stirring
the solution (at 1000 rpm), as compared to the response in
quiescent media. Kinetic control of the biocatalytic reaction
accounts for this behavior, which is attractive for minimizing the
effect of fluctuations in the natural convection under remote
conditions. FIG. 10 displays the dependence of the operating
potential upon the response to 5.times.10.sup.-6 M phenol.
Reductive detection of the liberated quinone product started at
+0.2V, rose sharply up to -0.05V, and decreased above -0.1V.
Potential steps to 0.0V were employed in all subsequent work, as a
compromise between sensitivity and selectivity. Such operating
potential eliminated possible contributions from coexisting
electroactive constituents.
The inherent selectivity and sensitivity of the probe led to
convenient quantitation of phenolic substrates in relevant
environmental samples. FIG. 11 displays the response to untreated
river (A) and groundwater (B) samples containing increasing levels
of cresol and phenol, respectively. A favorable response was
observed in both samples to these 2.5.times.10.sup.-6 M
concentration changes (b-i). The corresponding background
chronamperograms (a) indicated negligible contributions from
electroactive matrix constituents. Apparently, the biological
recognition, coupled with the low potential operation, allows
"fishing out" the target phenolic analytes from complex
environmental matrices. High sensitivity was also indicated from
the well-defined response and resulting calibration plots. A
similar performance was observed in analogous measurements of
phenol in river water and cresol in groundwater.
Remote environmental sensors need to rapidly respond to sudden
changes in the analyte concentration and offer a highly stable
response. A near real-time monitoring was obtained by carrying the
measurement at 1 minute intervals (in accordance to the
chronamperometric protocol). As indicated from FIG. 12(B), the
enzyme probe responds rapidly to dynamic changes in the phenol
concentration. No apparent carry over was observed between river
water samples spiked with 5.times.10.sup.-6 M and 2.times.10.sup.-5
M phenol. Notice also the reproducibility of the 12 runs at the
5.times.10.sup.-6 M level. The protocol and data of FIG. 12(B) show
how pre- and post-field calibrations can be utilized. The long term
stability is illustrated in FIG. 12(A) from repetitive
chronamperograms (for a river water sample containing
2.times.10.sup.-5 M phenol) recorded at 10 minute intervals over a
prolonged (500 min) period. A relative standard deviation of 2.4%
was calculated for the 50 runs of this series. Similar stability
(rsd=2.6%) was observed for analogous assays of a groundwater
sample.
Despite the absence of a protective membrane, no apparent sensor
passivation (e.g., surface fouling by matrix constituents) was
observed in these experiments. In addition, the biocatalytic
activity was maintained over long periods (e.g., ca. 80% after 2
weeks), in accordance with the known stability of tyrosinase (see
M. Bonakdar, Electroanal. Chem., 1989, 266, 47; J. Wang, Analyst,
1994, 119, 455; L. Campanella, Analyst, 1993, 118, 979; F. Ortega,
J. Pharm. Bioomed. Anal., 1992, 10, 789; J. Wangsa, Anal. Chem.,
1988, 60, 1080). Whenever needed, the probe design permitted easy
and fast replacement of the enzyme-electrode tip.
The above examples illustrate the ability to employ enzyme
electrodes for monitoring organic compounds at large
sample/instrument distances. The remote monitoring capability is
coupled to a selective, sensitive and reversible response.
Down-hole well monitoring and cone penetrometer insertions can also
be made.
The preceding examples can be repeated with similar success by
substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples. Some of the discussion of the metal sensor and
biosensor is applicable to the other sensor, particularly the
connector and housings.
Although the invention has been described in detail with particular
reference to these preferred embodiments, other embodiments (e.g.
bioelectrodes, modified electrodes) can achieve the same results.
Variations and modifications of the present invention will be
obvious to those skilled in the art and it is intended to cover in
the appended claims all such modifications and equivalents. The
entire disclosures of all references, applications, patents, and
publications cited above, and of the corresponding application(s),
are hereby incorporated by reference.
* * * * *